Split out OnDeviceModelAssetManager.

Move all of the on-device asset observation logic out of
ModelExecutionManager.  No functional changes.

Change-Id: Ie18d6d04cb6ab175fe956a83f4bde4c1a3ac5d02
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6115297
Reviewed-by: Zekun Jiang <zekunjiang@google.com>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Zekun Jiang <zekunjiang@google.com>
Auto-Submit: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1400550}